This invention relates to an emergency reactor core cooling structure adapted to operate when a coolant is lost due to the breakage of a pipe in a nuclear reactor, and more particularly, to an emergency reactor core cooling structure having a high operational reliability and being suitable for cooling a reactor core.
A conventional pressurized water reactor comprises a pressure vessel, a shroud disposed in the pressure vessel to form a downcomer, a core surrounded by the downcomer and constituted of a lot of fuel assemblies, a lower plenum on the lower side of the core, an upper plenum on the upper side of the core, and cold and hot legs secured to the pressure vessel and communicating with the downcomer and the upper plenum, respectively.
During the normal operation of this nuclear reactor, cooling water driven by a pump flows from the cold leg in the downward direction in the downcomer, and is guided into the core through the lower plenum. The cooling water is heated with the heat from the fuel in the core, and then guided into a steam generator via the upper plenum and the hot leg. The cooling water, the heat of which has been taken in the steam generator, returns to the pump and is driven thereby again into the cold leg. A nuclear reactor in the 1,100,000 kW-class has four such loops.
When a pipe, for example, the cold leg in such a pressurized water reactor is broken, the coolant flows out from a rift, that is, a loss of coolant takes place, so that the water level in the pressure vessel lowers. In order to reduce the quantity of heat generated due to the nuclear fission in the core, control rods are inserted into the fuel assemblies through upper core support columns to cause a scram of the core to occur. Even after the scram of the core has occurred, decay heat is generated in the core, and the water level in the core lowers. Consequently, the fuel assemblies are exposed, and the temperature thereof then increases. The emergency reactor core cooling structure is then operated so that the cooling water is injected from the cold and hot legs into the interior of the pressure vessel to cool the core. During this time, the vapor generated in the core due to the decay heat and the vapor generated in the lower plenum due to the depressurization boiling of the water flow up to an upper core plate partitioning the core and the upper plenum to suppress the falling of the cooling water through bores therein (a CCFL phenomenon). Therefore, a part of the cooling water supplied from the hot leg flows into the loop having a rift, and flows out therefrom. In the meantime, the pressure in the core increases due to the hydrostatic head of the cooling water stored in the upper plenum and a local pressure loss into the core at the upper core plate, and a submergence speed at which the cooling water, injected from the cold leg into the lower plenum through the downcomer flows up in the core is limited.
When such a loss of a coolant occurs in a nuclear reactor provided with a conventional emergency core cooling structure, the cooling water held on the outer side of the pressure vessel is injected into the core through upper control rod guide pipes as described in Japanese Patent Laid-open No. 43396/1975. However, no consideration is given to the effective utilization of the cooling water, which is held on the upper side of the core by the vapor flowing up from the core, for the purpose of cooling the core. A conventional emergency core cooling structure designed to cool the fuel assembly in a boiling water reactor is provided, on the upper side of the core, with a core spray and a cooling water guide pipe which has a funnel type opening opened on the upper side of the fuel assembly and a plurality of openings in the side wall of the same pipe as shown in Japanese Patent Laid-open No. 56298/1977. However, no consideration is given to a counter-current flow limiting phenomenon (CCFL phenomenon) which occurs due to the vapor flowing up in the cooling water guide pipe. Therefore, as shown in Japanese Patent Laid-open Nos. 59293/1977 and 59294/1977 which disclose the provision of a core sprayer on the upper side of the core, and a guide pipe having a plurality of openings in the side wall thereof and an opening at the upper end thereof, each of the cooling water guide pipes in these prior art emergency core cooling structures serves as a pipe for guiding the vapor, which flows from the openings in the side wall thereof into the same pipe, so as to be discharged to the space on the upper side of the fuel assembly through the same guide pipe. Since the flow rate of the vapor flowing out from the vapor guide pipe is limited, so that the guide pipe does not contribute to the decrease in the quantity of vapor flowing up to the upper tie plate on the upper side of the fuel assembly. Namely, the guide pipe has little effect in introducing the cooling water on the upper side of the fuel assembly into the interior thereof. In order that the cooling water on the upper side of the core or fuel assembly is dropped effectively into the core or the interior of the fuel assembly, it is necessary to consider the mutual effects of the vapor flowing up in the interior of the guide pipe and that of the vapor flowing up in the exterior thereof with respect to a counter-current flow limiting phenomenon.
In the above-described conventional emergency core cooling structures, no consideration is given to the techniques for promoting the falling of the cooling water against the counter-current flow limiting phenomenon occurring in the cooling water held on the upper side of the core or fuel assembly by the vapor flowing up from the interior of the core or the fuel assembly. In order to supply cooling water from the outside of the pressure vessel into the interior thereof by a core spray, for example, a means for detecting the breakage of a pipe and a cooling water supply means consisting of some constituent parts, such as a pump and a valve are required. This causes the construction of emergency core cooling structure to be complicated.